Flexible transparent conductive film is one of critical materials for flexible optoelectronic devices, such as flexible display panels, touch control panels, electronic papers, solar cells and the like. At present, the commercial flexible transparent conductive film is mainly a composite film formed by sputtering indium tin oxide (ITO) compound on a transparent polymer film. On one hand, indium is a precious metal, which belongs to scarce resource; and on the other hand, ITO film has high brittleness and poor flexibility, and the ITO-based transparent conductive film has short bending fatigue life and can hardly meet the requirements to optoelectronic development. Many institutions and companies work on developing flexible transparent conductive films free of ITO.
When the thicknesses of metal films, such as silver, gold, aluminum and the like, are less than 20 nm, the metal films are still electrical conductive. But the electrical conductivity of the metal films decreased with the decrease of the thickness, while absorption and reflection of visible light decreased sharply, so that the nano metal film may present certain light transmission and electrical conductivity. However, the cost to fabricate nano metal films on polymers is very high, hence, the flexible transparent conductive nano metal film is difficult to be produced in large scale and commercialized. It is another choice to prepare flexible transparent conducive films by forming a conductive network of metal nanowires or metal nanoparticles. ClearOhm technology is to prepare silver nanowire and silver nanowire ink, which was developed by Cambrios company in America in 2008 and was used to fabricate flexible transparent conducive films. The silver wire has a diameter of about 100 nm, and an aspect ratio greater than 300. The silver nanowire ink was coated on a transparent polymer film, and the silver nanowires in random orientation form a conductive network. The sheet resistance of the obtained flexible films can be 50-300Ω/□ and the light transmittance is about 92%. The key to this technology is controlling the distribution of the metal nanowires on the substrate to form conductive paths. In order to promote the application of silver nanowire ink on touch control panels, Cambrios and others investigated the feasibility of patterning the FTCF with silver nanowire networks, such as by laser ablation technique.
Poly(3,4-ethylenedioxythiophene)(PEDOT) is an intrinsic conducting polymer with excellent electrical conductivity, optical transparency and environmental stability. It is a good candidate to replace ITO in flexible transparent conducive films. However, PEDOT is insoluble and infusible. It can hardly be processed into a thin film. The aqueous dispersion of PEDOT and polystyrenesulfonate (PSS) complex, i.e., Baytron P developed by Bayer A G, may be coated in thin film, thereby solve the problem of forming PEDOT film in a certain degree. However, the insulated PSS layer hinders charge migration; the electrical conductivity of PEDOT/PSS film is low; also, the moisture absorption of PSS component is high. The electrical conductivity and reliability of the PEDOT/PSS film cannot meet the market requirements. Many researchers investigate in-site polymerization of EDOT monomers on substrates to solve the film-forming problem and obtain transparent PEDOT film with higher electrical conductivity and stability. The explored methods include direct polymerization, adsorption in polymerizing solution, chemical vapor deposition (CVD), vapor phase polymerization (VPP) and liquid phase depositional polymerization (LPDP).
The method of direct polymerization comprises the steps: mixing 3,4-ethylenedioxythiophene (EDOT) monomer and oxidant solutions together, then coating the mixture on a substrate and finally heating it for chemical oxidative polymerization of the monomer. In the direct polymerization, almost no solvent remained in the middle period of the reaction, hence it is difficult for EDOT oligomers to move and join each other; the obtained PEDOT have low polymerization degree and poor regularity. The electrical conductivity of the obtained PEDOT films can approach 100 S/cm. Moreover, once the monomer and oxidant solution are mixed, the chemical oxidative polymerization is started. Although retardants, such as imidazole and other organic bases, may prolong the pot life of the mixture, the repeatability of the experiment is extremely poor and the electrical conductivity may fluctuate in several magnitudes.
The method of adsorption in polymerizing solution comprises the steps: laying a substrate in the bottom of EDOT monomer solution; and adding oxidant solution for chemical oxidative polymerization; thus, same synthesized PEDOT molecules coalesce in the solution and are deposited on the substrate to form a PEDOT coating. The PEDOT film obtained by this method is low in compactness and adhesion. Also, the utilization of monomer is extreme low. Though sulfonic groups grafted on substrates can increase the adhesion of the PEDOT film, other drawbacks remain still.
The method of chemical vapor deposition (CVD) comprises the steps: loading a substrate in a reaction chamber and simultaneously introducing oxidant and monomer vapors into the reaction chamber. The monomer and oxidant vapors deposit on the substrate and oxidative polymerize. After washing off the oxidant residues and EDOT oligomers, transparent PEDOT film with electrical conductivity above 1000 S/cm can be prepared. However, special equipment is needed for CVD; also, the operation requirement is high; and the types of optional oxidants are limited. CVD is unsuitable for production in large-scale.
The method of vapor phase polymerization (VPP) comprises the steps: pre-forming an oxidant coating on substrate and exposing the substrate with oxidant coating to monomer vapor. The monomer vapors deposit on the oxidant coating continually and polymerize in situ. Kim et al exposed a PET film with FeCl3 and surfactant to EDOT vapor. As monomer vapor deposited on the PET film and polymerized to form PEDOT film, thereby, transparent PET composite film with sheet resistance of 500Ω/□ was prepared. The acidity of solid ferric salt oxidant is high enough to catalyze the side reactions of addition polymerization of EDOT, resulting in conjugative defects in the PEDOT chains and nonconductive products. The side reactions may be inhibited by doping volatile organic bases into the oxidant. Also, in VPP, moisture does benefit the deprotonation of cationic EDOT dimers and construct conjugative PEDOT molecules. However, moisture will stimulate the hydration, and crystallization of ferric salt oxidants and reduce the reactive activity of oxidants. This may lead to pinholes on the synthesized PEDOT film. In VPP, only chamber humidity, oxidant formula and reaction conditions are controlled tightly, high quality PEDOT coating could be produced.
The method of liquid phase depositional polymerization is suspending substrate with oxidant coating in EDOT solution to synthesize PEDOT coating in-situ, in order to overcome the drawbacks of VPP, Li et al suspended substrates with oxidant coating in EDOT solution and PEDOT coating was synthesized on the substrate in situ. Furthermore, they introduced peroxy acid into the surface of the substrate as second oxidant to solve the problem of ferric salt loss at high concentration and enhance the efficiency of coated ferric salt oxidant.
In order to improve the electrical conductivity of PEDOT coating, some researchers combined the nano silver with PEDOT coating. They added silver nanowires or silver nanoparticles into PEDOT/PSS dispersion so as to prepare a PEDOT/nano silver hybrid film.
As mentioned above, at present, commercial flexible transparent conductive films mainly refer to polymeric films with ITO coating; they have high price and poor bending resistance. Non-ITO substitutes are being investigated. Particularly, conducting PEDOT films and silver nanowire ink have high competitiveness and ability to market penetration. The present invention provides a technology for in-site synthesis of transparent conductive PEDOT/nano silver coating on transparent substrates and a method for fabricating flexible transparent conductive film.